1. Field of the Invention
The invention relates to a driving method for plasma display panels used for a flat screen type television, an information display and the like, in particular, to a driving method which can reduce false contours in dynamic images in a sub-field method.
2. Description of the Related Art
Plasma display panels (xe2x80x9cPDPxe2x80x9d) have many advantages such that they have thinner structures, provide less flickers and larger display contrasts, can be easily built into large screen sizes, have faster response speeds, and are capable of multicolor luminance by using phosphors as they are emissive type. Consequently, they are becoming quite popular as computer-related displays and other color image displays.
The PDP can be divided based on their operating principles into an AC type one, which operates under an alternating current discharge state indirectly with the electrodes covered with dielectric materials, and a DC type one, which operates under a direct current discharge state with the electrodes being exposed to a discharge space. Moreover, the AC type PDP can be divided depending on the driving method into a memory operating type, which uses a memory function of each discharge cell, and a refresh operating type, which doesn""t use the function. The intensity of a PDP is proportional to the number of discharges. Since the intensity of the refresh type reduces as the display capacity increases, it is primarily used as a small display capacity PDP.
FIG. 1 is a perspective drawing showing the structure of display cells of a typical AC type PDP, and FIG. 2 is a cross-sectional drawing of the same.
A display cell 16 has two insulating substrates 1 and 2 made of glass. The insulating substrate 1 is a back substrate and the insulating 2 is a front substrate.
One side of the insulating substrate 2 that is facing the insulating substrate 1 is provided with transparent scanning electrodes 3 and transparent sustaining electrodes 4. The scanning electrodes 3 and the sustaining electrodes 4 extend in the horizontal direction (transverse direction) of the panel. Bus electrodes 5 and 6 are arranged so as to overlay upon the scanning electrodes 3 and the sustaining electrodes 4, respectively. The bus electrodes 5 and 6 can be made of a metal and are provided there to minimize the electrical resistance value between each electrode and the external drive unit. A dielectric material layer 12 is provided to cover the scanning electrodes 3 and the sustaining electrodes 4. Also provided is a protective layer 13 made of magnesium oxide or the like for the purpose of protecting the dielectric layer 12 from discharges.
A data electrode 7 is provided in a direction perpendicular to those of the scanning electrodes 3 and the sustaining electrodes 4 on one side of the insulating substrate 1 that faces the insulating substrate 2. Thus, the data electrode 7 extends in the perpendicular direction (vertical direction) of the panel. Partition walls 9 are separating the display cells in the horizontal direction. A dielectric layer 14 is provided to cover the data electrodes 7. A phosphor layer 11 is formed in such a way as to cover the side surfaces of the partition walls 9 and the surface of the dielectric layer 14 in order to convert an ultraviolet light generated by a discharge of discharge gas into visible light 10. A discharge gas space 8 is secured with the partition walls 9 between the insulating substrates 1 and 2, and discharge gas containing helium, neon, xenon or the like, or a mixture of these gases fills the discharge gas space 8.
The discharge operation will be described that occurs in a display cell 16 selected on a conventional PDP constituted as described above.
When discharge starts as the pulse voltage higher than the discharge threshold value is applied between the scanning electrode 3 and the data electrodes 7, positive and negative charges will be attracted to the surfaces of the dielectric layers 12 and 14 and electric charges will accumulate in response to polarity of the pulse voltage. An equivalent internal voltage caused by the accumulation of charges, i.e., the wall voltage, has a polarity opposite to that of the pulse voltage. Thus, with the progress of the discharge, the effective voltage inside the cell drops, and will become impossible to sustain the discharge even if the pulse voltage is maintained at a constant value, and eventually the discharge will stop.
However, when a sustaining discharge pulse, which has the same polarity as the wall voltage, is applied between adjacent scanning electrodes 3 and sustaining electrodes 4, the wall voltage will be added to them as the effective voltage. Therefore, even if the voltage amplitude of the sustaining discharge pulse is respectively low, the effective voltage will exceed the discharge threshold value, and discharge will occur. Consequently, by applying a sustaining discharge pulse between the scanning electrodes 3 and the sustaining electrodes 4 reciprocally, the discharge becomes sustainable. This is the abovementioned memory function.
Moreover, by applying to the scanning electrodes 3 or the sustaining electrodes 4 an erasure pulse such as a wide and low voltage pulse that neutralizes the wall voltage, or a narrow width pulse with a voltage comparable to the sustaining discharge pulse voltage, the sustaining discharge can be stopped.
FIG. 3 is a block diagram showing the outline of a PDP formed by arranging display cells such as the one shown in FIG. 2 in a matrix as well as control circuits and drivers for the PDP.
A PDP 15 is a dot matrix display panel where display cells 16 typically shown in FIG. 2 in a matrix of m-rows and n-columns. Scanning electrodes X1, X2, . . . , Xm and sustaining electrodes Y1, Y2, . . . Ym are arranged parallel to each other as row electrodes, and data electrodes D1, D2, . . . , Dn are arranged perpendicular to the scanning electrodes and the sustaining electrodes as column electrodes.
A control circuit 31 is equipped with a frame memory 32 that stores image data for sub-fields that constitute a frame. It is also equipped with a signal processing memory control circuit 33 that receives vertical synchronous signal Vsync, horizontal synchronous signal Hsync, clock signal Clock and data DATA to read the data for sub-fields in the frame memory 32 based on those signals. The vertical synchronous signal Vsync is a signal to instruct the cycle for one frame and the starting point on the display screen for the cycle. For example, in case of constituting a frame asynchronous with the clock signal Clock, the vertical synchronous signal Vsync is to designate the leading display data DATA for the entire screen. The horizontal synchronous signal Hsync is a signal that instructs capture of display data for each horizontal scanning line. The horizontal synchronous signal Hsync corresponds to a signal that instructs the start of scanning for each horizontal scan in a cathode ray tube (CRT) display. A driver control circuit 34 is also provided for controlling the operation of the PDP 15 in relation to the output signal of the signal processing memory control circuit 33.
Further provided here a scan driver 21 that generates a scanning electrode drive pulse based on control signals received from the control circuit 31 and applies it to the scanning electrodes X1, X2, . . . , Xm, a sustain driver 22 that generates a sustaining electrode drive pulse based on control signals received from the control circuit 31 and applies it to the sustaining electrodes Y1, Y2, . . . Ym, and an address driver 20 that generates a data electrode drive pulse based on control signals received from the control circuit 31 and applies it to the data electrodes D1, D2, . . . , Dn.
Next, the prior driving method for the PDP shown in FIG. 3 will be described. FIG. 4 is a conceptual drawing showing one frame in the prior driving method, and FIG. 5 is a timing chart showing waveforms of the drive pulses outputted from the address driver 20, the scan driver 21 and the sustain driver 22 within one sub-field. In FIG. 5, Wu represents the sustaining electrode drive pulse applied to the sustaining electrodes Y1, Y2, . . . Ym, Ws1, Ws2, . . . , Wsm represent the scanning electrode drive pulses applied to the scanning electrodes X1, X2, . . . , Xm respectively, and Wd represents the data electrode drive pulse applied to the data electrodes Di (1xe2x89xa6ixe2x89xa6n).
As shown in FIG. 4, one frame consists of eight sub-fields SF1 through SF8, for example, and one sub-field (one cycle of drive) consists of four periods, i.e., a priming discharge period, a writing discharge period, a sustaining discharge period, and an erasure discharge period, and the cycle is repeated to obtain the display of a desired image.
The priming discharge period is a period for generating active particles and wall charges in the discharge gas space in order to achieve a stable writing discharge characteristic during the writing discharge period. During the priming discharge period, first of all, a priming discharge pulse Pp is applied to the scanning electrodes X1, X2, . . . , Xm to cause discharges at all of display cells in the PDP 115 as shown in FIG. 5. Next, the voltage level of the sustaining electrodes Y1, Y2, . . . Ym is raised to the sustaining voltage level Vs and a priming discharge erasure pulse Ppe is applied to the scanning electrodes X1, X2, . . . , Xm to bring down their voltages gradually in order to cause the erasure discharges. As a result, the portion of the wall charges thus created that hinders subsequent writing discharges and sustaining discharges to be erasure. The xe2x80x9cerasure of wall chargesxe2x80x9d mentioned here means not only the erasure of entire wall discharges but also includes the xe2x80x9cadjustment of amount of wall chargesxe2x80x9d for conducting the subsequent writing discharges and sustaining discharges smoothly.
During the writing discharge period, a certain scanning base voltage Pwb is applied to the scanning electrodes X1, X2, . . . , Xm, and a scanning pulse Pw is applied as well sequentially from the top of the scanning electrodes X1, X2, . . . , Xm. A data pulse Pd is applied selectively in synchronization with the scanning pulse Pw to the data electrodes Di (1xe2x89xa6ixe2x89xa6n) in the display cells that are to make displays. Consequently, writing discharges occur and wall charges are accumulated at the cells that are to make displays.
During the sustaining discharge period, a sustaining discharge pulse Pc is applied to the sustaining electrodes Y1, Y2, . . . Ym, while a sustaining discharge pulse Ps, whose phase is 180 degrees lagging that of the sustaining discharge pulse Pc, is also applied to the scanning electrodes X1, X2, . . . , Xm, so that the sustaining discharges can be repeated in order to obtain a desired intensity for each sub-field of the display cells where the writing discharges occur during the writing discharge period.
In the final erasure discharge period, erasure discharges are generated as the erasure pulse Pe is applied to the scanning electrodes X1, X2, . . . , Xm in order to drop their potentials slowly. As a result, the wall charges accumulated by the application of the sustaining discharge pulses Pc and Ps are erased. The xe2x80x9cerasure of wall chargesxe2x80x9d mentioned here also means not only the erasure of entire wall discharges but also includes the xe2x80x9cadjustment of amount of wall chargesxe2x80x9d for conducting the subsequent writing discharges and sustaining discharges smoothly.
The driving method described above is used for conducting displays on a typical AC type PDP of the prior art. The intensity in black display (background intensity) is determined relative to luminance with the priming discharge. There is a priming charging period exists for each sub-field in the driving method, so that luminance occurs a plurality of times due to the priming discharge unrelated to the display image during one frame period. As a result, it has a problem that the background intensity increases and causes a degradation of contrast if the number of sub-fields is increased in order to increase the number of gradations or picture quality.
In order to cope with this problem, another driving method for a PDP was proposed to achieve reductions of the priming discharge period and the erasure discharge period during one frame (Japanese Patent No. 2639311). Hereinafter, this driving method will be called a first prior art. FIG. 6 is a conceptual drawing of one frame according to a driving method for a PDP according to the first prior art.
In the first prior art, a sub-field group is formed of arranging a plurality of sub-fields of the same luminance intensity, a group containing sub-fields SF9, SF8 and SF7, another group containing sub-fields SF6, SF5 and SF4, another group containing sub-fields SF3, SF2 and SF1. Writing and erasure of each display cell are provided only once for a single sub-field group and a priming discharge period is provided between the sub-field groups.
This reduces the priming discharge frequency, thus reducing the background intensity and improving the display contrast.
Also, a driving method of providing the lowest sub-field (LSF) immediately after the highest sub-field (MSF) without providing the priming discharge period between them, and a driving method of providing a sub-field one level higher than the lowest sub-field immediately after a sub-field one level lower the highest sub-field without providing the priming discharge period between them (Japanese Unexamined Patent Publication No. Hei 9-319330). Hereinafter, the former driving method disclosed in the Japanese Unexamined Patent Publication No. Hei 9-319330 will be called a second prior art. FIG. 7 is a conceptual drawing of one frame according to the driving method of a PDP according to the second prior art.
If the highest sub-field (MSF) is chosen in the second prior art, the lowest sub-field (LSF) will automatically be selected. As a result, the number of writing erasure periods per one frame will be reduced.
However, although the original object is achieved in the first prior art, it has a problem that it is difficult to realize a more sophisticated display device since the flexibility is limited as the luminance intensities of the sub-fields are constant within one sub-field group. For example, a problem occurs when the sustaining discharge frequency within one frame is switched according to the signal level.
Since the intensity of the PDP depends on the sustaining discharge frequency, the improvement of the intensity is realized by increasing the number of sustaining pulses. The power consumption becomes too large if it is tried to achieve a sufficient intensity in such a PDP. A peak intensity enhancing control method is known, in which the power consumption is lowered by reducing the sustaining discharge frequency if the average picture level of the image signal is high, and the sustaining discharge frequency is increased in order to achieve a high contrast feeling by increasing the intensity of small regions if the average picture level is low.
For example, if the peak intensity enhancing control method is applied to the first prior art by assuming that the number of sub-fields in each sub-field group is two, the minimum number of sustaining discharges that are adjustable relative to the average picture level change is two. This is due to the fact that reducing the number of sustaining discharges one at a time becomes smallest in order to have uniform sub-field luminance intensity within a sub-field group. Consequently, one step of display intensity variation according to the signal level change may become too large in some cases, and this step of intensity variation may become unpleasant for the viewer.
A major problem in gradation expression based on the sub-filed method is in general that the contour interference occurs when dynamic images are displayed. This interference is generally called xe2x80x9cdynamic image false contourxe2x80x9d and is caused by the fact that the regularity of the luminance period varies when the display image moves. The sub-field method is a method of providing a plurality of sub-fields weighted with various intensities and changing the combination of the selected sub-fields in order to change the average intensity within a frame. Human eyes feel the integrated value of the luminance within a frame in the frame frequencies of 60 Hz or higher, at which flickers cannot be detected as the intensity of the display image. However, if there is a movement in an image with intermittent luminance, human eyes tend to follow the image as a habit and, when an irregularity of intermittent luminance appears at a place where the image existed in the previous frame, the person feels it brighter if the interval is short and darker if the interval is longer than the actual change of the average intensity of the frame.
There has been many driving methods proposed to correct dynamic image false contours, among which a driving method called the redundancy coding method has been known to be particularly effective, wherein a large number of sub-fields are used to have a large number of sub-field combinations to correspond with a signal level. In order to apply the first prior art to this redundancy coding method, it will be necessary to further increase the number of sub-fields. However, there is a limit for elongating the duration of one frame as it is necessary to maintain the frequency within the range (higher than approximately 60 Hz) which human eyes do not detect flickers. Therefore, there is a practical limit for increasing the number of sub-fields. Even if it is possible to increase it, it will cause a problem that the sustaining discharge period becomes insufficient and intensity decreases as a result.
In the second prior art, on the other hand, there is a problem that the lowest sub-field is automatically chosen with the selection of the higher sub-field, so that the minimum change of the gradation level in an image expressed with signals of higher intensity level increases and the number of practical gradation steps decreases.
The object of the present invention is to provide a driving method for plasma display panels capable of maintaining high contrasts by reducing background intensity and simultaneously reducing dynamic image false contours by suppressing the variation of the image intensity.
According to one aspect of the present invention, a driving method for plasma display panel wherein gradation is expressed by dividing one frame into a plurality of sub-fields comprises the steps of conducting sustaining discharges in a first sub-field in a pair of adjacent first and second sub-fields, and conducting writing discharges in the second sub-field after the sustaining discharges in the first sub-field without conducting any erasure discharge between the sustaining discharges in the first sub-field and the writing discharges in the second sub-field. A relation expressed by an equation L1=L2=1 and an inequality Ln+2xe2x89xa6Ln+1+Ln holds for a luminance weighting Li. The luminance weighting Li is a luminance weighting of the i-th lowest sub-field from the bottom among the plurality of sub-fields.
In the present invention, writing discharges for the second sub-field, which generates luminance later, are conducted immediately after conducting sustaining discharges for the second sub-field, which generates luminance earlier among a pair of adjacent first and second sub-fields. Therefore, there exist some sub-fields having no priming discharge. Thus, it is possible to sustain contrasts higher by reducing the background luminance. Also, since the weighting of intensity in each sub-field is only required to satisfy the relation expressed by the equation L1=L2=1 and the inequality Ln+2xe2x89xa6Ln+1+Ln, many gradation levels exist where many combinations of sub-fields that express the gradation levels exist. Therefore, it is possible to suppress the variation of the image intensity by increasing the redundancy and reduce dynamic false contours by suppressing one step of the intensity variation.
According to another aspect of the invention, a driving method for plasma display panel wherein gradation is expressed by dividing one frame into a plurality of sub-fields comprises the steps of preparing frames in each of which number of priming discharge periods is set up in relation to an average picture level of images, the number of sub-fields in the frames being equal to one another, selecting a frame from the frames in accordance with a average picture level, conducting sustaining discharges in a first sub-field in a pair of adjacent first and second sub-fields, and conducting writing discharges in the second sub-field after the sustaining discharges in the first sub-field without conducting any erasure discharge between the sustaining discharges and the writing discharges in case when the number of priming discharge periods in a selected frame is less than the number of sub-fields in the selected frame.